The purchase application index has been highlighted as a particularly important data series as it very broadly captures the demand side of residential real estate for both new and existing home purchases.

The latest data is showing that the average rate for a 30 year fixed rate mortgage declined 10 basis points since the last week to 4.59% while the purchase application volume increased 3.4% and the refinance application volume increased 8.6% over the same period.

It's important to note that with the final expiration of the governments massive housing tax credit subsidy, home purchase activity has been trending down precipitously despite falling interest rates.

The purchase application volume is now near the lowest level seen in well over a decade.

The following chart shows how the principle and interest cost and estimated annual income required to cover the PITI (using the 29% “rule of thumb”) on a $400,000 loan has changed since November 2006.

The following chart shows the average interest rate for 30 year and 15 year fixed rate mortgages over the last number of weeks (click for larger version).

The following charts show the Purchase Index, Refinance Index and Market Composite Index since November 2006 (click for larger versions).